J.R.R.Tolkien, written c. 1958, edited and published by Christopher Tolkien in The History of Middle Earth volume X: Morgoth's Ring, Houghton Mifflin, 1993.

Excerpts follow:

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Sauron was ‘greater’, effectively, in the Second Age than Morgoth at the end of the First. Why? Because, though he was far smaller by natural stature, he had not yet fallen so low.

Eventually he also squandered his power (of being) in the endeavor to gain control of others. But he was not obliged to expend so much of himself. To gain domination over Arda, Morgoth had let most of his being pass into the physical constituents of the Earth – hence all things that were born on Earth and live on and by it, beasts or plants or incarnate spirits, were liable to be ‘stained’.

Morgoth at the time of the War of the Jewels had become permanently ‘incarnate’: for this reason he was afraid, and waged the war almost entirely by means of devices, or of subordinates and dominated creatures.

Sauron, however, inherited the ‘corruption’ of Arda, and only spent his (much more limited) power on the Rings; for it was the creatures of earth, in their minds and wills, that he desired to dominate. In this way Sauron was also wiser than Melkor-Morgoth. Sauron was not a beginner of discord; and he probably knew more of the ‘Music’ than did Melkor, whose mind had always been filled with his own plans and devices, and gave little attention to other things.

The time of Melkor’s greatest power, therefore, was in the physical beginnings of the World; a vast demiurgic lust for power and the achievement of his own will and designs, on a great scale. (…)

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Thus, as ‘Morgoth’, when Melkor was confronted by the existence of other inhabitants of Arda, with other wills and intelligences, he was enraged by the mere fact of their existence, and his only notion of dealing with them was by physical force, or the fear of it. His sole ultimate object was their destruction.

Elves, and still more Men, he despised because of their ‘weakness’: that is their lack of physical force, or power over ‘matter’; but he was also afraid of them. He was aware, at any rate originally when still capable of rational thought, that he could not ‘annihilate’them: that is, destroy their being; but their physical ‘life’, and incarnate form became increasingly to his mind the only thing that was worth considering.

Or he became so far advanced in Lying that he lied even to himself, and pretended that he could destroy them and rid Arda of them altogether. Hence his endeavor always to break wills and subordinate them to or absorb them into his own will and being, before destroying their bodies. This was sheer nihilism, and negation its one ultimate object:

Morgoth would no doubt, if he had been victorious, have ultimately destroyed even his own ‘creatures’, such as the Orcs, when they had served his sole purpose in using them: the destruction of Elves and Men. (…)

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Melkor could do nothing with Arda, which was not from his own mind and was interwoven with the work and thoughts of others: even left alone he could only have gone raging on till all was leveled again into a formless chaos. And yet even so he would have been defeated, because it would still have ‘existed’, independent of his own mind, and a world in potential.

Sauron had never reached this stage of nihilistic madness. He did not object to the existence of the world, so long as he could do what he liked with it. He still had the relics of positive purposes, that descended from the good of the nature in which he began: it had been his virtue (and therefore also the cause of his fall, and of his relapse) that he loved order and co-ordination, and disliked all confusion and wasteful friction.

(It was the apparent will and power of Melkor to effect his designs quickly and masterfully that had first attracted Sauron to him.)

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Sauron had, in fact, been very like Saruman, and so still understood him quickly and could guess what he would be likely to think and do, even without the aid of the palantíri or of spies; whereas Gandalf eluded and puzzled him.

But like all minds of this cast, Sauron’s love (originally) or (later) mere understanding of other individual intelligences was correspondingly weaker; and thought the only real good in, or rational motive for, all this ordering and planning and organization was the good of all the inhabitants of Arda (even admitting Sauron’s right to be their supreme lord), his ‘plans’, the idea coming from his own isolated mind, became the sole object of his will, and an end, the End, in itself.

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Morgoth had no ‘plan’: unless destruction and reduction to nil of a world in which he had only a share can be called a ‘plan’.

But this is, of course, a simplification of the situation. Sauron had not served Morgoth, even in his last stages, without becoming infected by his lust for destruction, and his hatred of God (which must end in nihilism).

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Sauron could not, of course, be a ‘sincere’ atheist. Though one of the minor spirits created before the world, he knew Eru, according to his measure. He probably deluded himself with the notion that the Valar (including Melkor) having failed, Eru had simply abandoned Ea, or at any rate Arda, and would not concern himself with it any more.

It would appear that he interpreted the ‘change of the world’ at the Downfall of Numenor, when Aman was removed from the physical world, in this sense: Valar (and Elves) were removed from effective control, and Men under God’s curse and wrath.

If he thought about the Istari, especially Saruman and Gandalf, he imagined them as emissaries from the Valar, seeking to establish their lost power again and ‘colonize’ Middle-earth, as a mere effort of defeated imperialists (without knowledge or sanction of Eru).

His cynicism, which (sincerely) regarded the motives of Manwë as precisely the same as his own, seemed fully justified in Saruman. Gandalf he did not understand. But certainly he had already become evil, and therefore stupid, enough to imagine that his different behaviour was due simply to weaker intelligence and lack of firm masterful purpose. He was only a rather cleverer Radagast – cleverer, because it is more profitable (more productive of power) to become absorbed in the study of people than of animals.

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Sauron was not a ‘sincere’ atheist, but he preached atheism, because it weakened resistance to himself (and he had ceased to fear God’s action in Arda). As was seen in the case of Ar-Pharazon.

But there was seen the effect of Melkor upon Sauron: he spoke of Melkor in Melkor’s own terms: as a god, or even as God. This may have been the residue of a state which was in a sense a shadow of good: the ability once in Sauron at least to admire or admit the superiority of a being other than himself. Melkor, and still more Sauron himself afterwards, both profited by this darkened shadow of good and services of ‘worshippers’.

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But it may be doubted whether even such a shadow of good was still sincerely operative in Sauron by that time.

His cunning motive is probably best expressed thus. To wean one of the God-fearing from their allegiance it is best to propound another unseen object of allegiance and another hope of benefits; propound to him a Lord who will sanction what he desires and not forbid it. Sauron, apparently a defeated rival for world-power, now a mere hostage, can hardly propound himself; but as the former servant and disciple of Melkor, the worship of Melkor will raise him from hostage to high priest.

But though Sauron’s whole true motive was the destruction of the Númenóreans, this was a particular matter of revenge upon Ar-Pharazon, for humiliation. Sauron (unlike Morgoth) would have been content for the Númenóreans to exist, as his own subjects, and indeed he used a great many of them that he corrupted to his allegiance.

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Note - Melkor could not, of course, ‘annihilate’ anything of matter, he could only ruin or destroy or corrupt the forms given to matter by other minds in their subcreative activities.

Note – [Sauron’s] capability of corrupting other minds, and even engaging their service, was a residue from the fact that his original desire for ‘order’ had really envisaged the good estate (especially physical well-being) of his ‘subjects’.

(...)

Melkor incarnated himself (as Morgoth) permanently. He did this so as to control the hroa, the flesh or physical matter, of Arda. He attempted to identify himself with it. A vaster, and more perilous, procedure, though of similar sort to the operation of Sauron with the Rings.

Thus, outside the Blessed Realm, all matter was likely to have a Melkor ingredient, and those who had bodies, nourished by the hora of Arda, had as it were a tendency, small or great, towards Melkor: they were none of them wholly free of him in their incarnate form, and their bodies had an effect upon their spirits.

But in this way Morgoth lost (or exchanged, or transmuted) the greater part of his original angelic powers, of mind and spirit, while gaining a terrible grip upon the physical world. For this reason he had to be fought, mainly by physical force, and enormous material ruin was a probable consequence of any direct combat with him, victorious or otherwise.

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This is the chief explanation of the constant reluctance of the Valar to come into open battle against Morgoth. Manwë's task and problem was much more difficult than Gandalf's.

Sauron's, relatively smaller, power was concentrated; Morgoth's vast power was disseminated. The whole of Middle-earth was Morgoth's Ring, though temporarily his attention was mainly upon the North-west. Unless swiftly successful, War against him might well end in reducing all Middle-earth to chaos, possibly even all Arda.

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It is easy to say: It was the task and function of the Elder King to govern Arda and make it possible for the Children of Eru to live in it unmolested. But the dilemma of the Valar was this: Arda could only be liberated by a physical battle; but a probable result of such a battle was the irretrievable ruin of Arda. Moreover, the final eradication of Sauron (as a power directing evil) was achievable by the destruction of the Ring. No such eradication of Morgoth was possible, since this required the complete disintegration of the matter of Arda.

The last intervention with physical force by the Valar, ending in the breaking of Thangorodrim, may then be viewed as not in fact reluctant or even unduly delayed, but timed with precision. The intervention came before the annihilation of the Eldar and the Edain.

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Morgoth though locally triumphant had neglected most of Middle-earth during the war; and by it he had in fact been weakened: in power and prestige (he had lost and failed to recover one of the Silmarils), and above all in mind.

He had become absorbed in kingship, and though a tyrant of ogre-size and monstrous power, this was a vast fall even from his former wickedness of hate, and his terrible nihilism.

He had fallen to like being a tyrant-king with conquered slaves, and vast obedient armies.